The purpose of this project is to understand the mechanisms by which milk-protein production in mouse mammary tissue is controlled by various hormones with particular emphasis on prolactin-thyroid hormone interactions. Studies include: 1) evaluation of the cytological and ultrastructural development of virgin mouse mammary tissue after explant culture in the presence of insulin, hydrocortisone, prolactin and thyroid hormones; 2) examination of biochemical and ultrastructural development of these tissues in the presence and absence of DNA synthesis in vitro; 3) characterization of thyroid hormone and prolactin receptors within mammary tissue and their interrelationship; 4) examination of the role that prostaglandins may play in milk-protein production with emphasis on regulation by thyroid hormones and prolactin.